Electrical heating unit



May 23, 1933. L. M. DALY ELECTRICAL HEATING UNIT Original Filed Nov. 30, 1929 Dale, W

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Patented May 2 3, 1933 UNITED STATES PATENT OFFICE LORETTA H. DALY, OF PITTSFIELD, MASSACHUSETTS, ASSIGNOR T0 GENERAL ELECTRIC COMPANY, A CORPORATION OF YORK ELECTRICAL nns'rmo .UNIT

Application flied November 30, 1929, Ser1a1 No. 410,828. Renewed October 28, 1982.

My, invention relates to electric heating units, more particularly to electric heating units of the sheath wire type, and has for its object the provision of an improved method of making an electric heating unit of this character.

More specifically my invention relates to electric heating units of the sheath wire t pc in which it resistance element is embedde in a compacted mass of powdered insulatin material enclosed by a metallic sheath, sue for exemple as described end claimed in the United States patent granted to C. C Ab hott, No, 1,367,341, dated February 1, 1921, My invention is especially directed to role tiveiy long electric hosting units of this type having s. coiied resistance element.

It has been very difiicult, heretofore, to moire a sheath wire heating unit, provided with u coiled resistence element, of relatively great length which will given uniform host, and especialiy so if the resistance element is formed from compar tiveiy small conductoiz This is due to the fact that the powdered insulating materiel While being loaded into the sheath tends to carry the coils of the resistance element with it endwise in the sheath. It will. be understood that during the loading operation, the sheath is held in an upright position and as the sheath is being filled with the insulating material through its upper end, it is vibrated so as to distribute the insulating material uniformly and to compact it about the resistance element. It is during this operation that the coils of the resistance element are carried endwise within the sheath by the insulating material.

It has been found in some instances that by reason of this action at least one-half of the total length of the resistance element was located /i n but one-third of the length of the finished heating unit.

In carrying my invention into efiect in one form thereof, I secure the resistance element at a plurality of points spaced at intervals I tion.

along its length so that the element will not be carried lengthwise in the sheath. by the insulating material during th loading opera- More specifically, I have provided insulating supports spaced at intervals along the len th of the resistance element and have reduced portions of the metallic sheath so as to prevent endwise movement of the supports and consequently oi the resistance element, The portions of the sheath to he reduced may be located conveniently by placing the resis ance element With its supports along the sheath. Then either the positions assumed by the supports or the positions of the sheath to be reduced me be indicated by suitably marking the sheetli.

For a more complete understanding or my invention reference should be had the accompanying drawing in which Fig, 1 is an elevation illustrating the coiled resistance element With the supports spaced intervals slung its length and secured. to the'element; Fig 2 is an elevation of the shesth illustra ing the sheath suitably merited so as to indicate the portions to be reduced; Fig 3 is an elevation partly in section illustrating the resistance element together with its supports mounted Within the sheath, and also illustrating the sheath reduced so as to'prevent endwise movement of the supports; Fig. l is an elevation partly in section illustrating the sheath after it has been filled with the powdered insulating material; Fig. 5 is on elevation illustrating the heating unit after the sheath has been reduced; and Fig. 6 is a pers ective view on an enlarged scale illustrating t e manner in which th insulating supports are secured to the resistance element.

Referring to the drawing, in one form of my invention the elongated heating unit 10 comprises a helical or sinuous resistance element 11 extending centrally of a metallic sheath 12. Preferably, the resistance element will be made of a suitable high resistance material having considerable resiliency,

such as nickel-chromium alloy. The heating element in the finished heating unit (Fig. 5) is embedded in a compacted mass of powdered heat refractory insulatin material 13, such as magnesium oxide. etallic terminals 14 and 15 are provided for the resistance element 11.

In forming the heating unit in accordance with my invention, insulating supports 16 are threaded on the resistance element 11, after which the terminals 14 and 15 are secured. Preferably, the terminals will be assembled and secured in a manner substantially the same as that described and claimed in the United States patent granted to C. C. Abbott No. 1,494,938, dated May 20, 1924. In accordance with themethod there described, each terminal is rovided with a threaded portion 17 on which the end portions of the resistance element 11 are turned or screwed. It will be understood that the diameter of the helix is slightly less than the diameter of the terminals at the roots of the threads so that the helix is expanded somewhat when it is turned on. The resistance conductorconsequently contracts by reason of its resiliency on the threaded portions thereby making a good electrical and mechanical connection with the terminals. $uitable insulating washers 18 are provided on the threaded ends of the terminals for holding the inner ends of the teminals centrally of the sheath when the heating element ismounted within the sheath. As shown, the terminals are provided with ears 19 for retaining the washers on one side, the washers being retained on the other side by the coiled resistance element. The outer end of the terminal 14 is provided with a solid loading washer 20 which is secured on the terminal by means of the flattened end terminal portion 14a, while the outer end of the terminal'15 is provided with a loading pin 21 which is passed throu h the flattened end terminal portion 15a 0 the terminal. This flattened end portion, of course, is provided with an aperture for receiving the pin.

After the terminals have been secured, the centering supports 16 are spaced along the lam th of the helical resistance element. It wil be understood that any suitable number of supports may be spaced on the element, the number to a large extent depending upon the size of the conductor from which the resistance element is formed and on the length of the heating unit. Eachsupport is secured on the resistance element by compressing or flattenin several turns of the conductor on each slde of and adjacent the support as is clearl shown in Fig. 6.

As shown (Fig 6), each support 16 comprises a cylindrical reduced body portion 16a and an enlarged flan e-like head portion 166 located at one end 0 the body rtion. The support is provided with a longitudinal passageway sufiiciently lar to provide a running fit with the coil. t will be understood therefore, that the turns on each side of the support need be flattened to but a sli ht degree in order to secure the support. he su ports will be formed from a suitable insu ating material such as lava. It will also be understood that the supports 16 may have other shapes. Thus, for example, sup orts havin square cross-sections may be use successfu ly.

As has been pointed out, I provide the metallic sheath with a plurality of reduced or flattened portions 12a located so as to secure the supports and consequently the resistance element against lengthwise movement during the process of loadin the sheath with the insulating material. locate the portions of the sheath to be reduced by placing the resistance element with its terminals and supports lengthwise of the sheath. The resistance element will be tensioned (Fig. l) to the length that it will assume when positioned in the sheath so that each support will assume a position relative to the length of the sheath, which will be substantiall the position relative to the length of the s eath that it will occupy when the element is positioned within the sheath. Those portions of the sheath which are to be reduced will be marked or indicated in some suitable manner as by chalking the outer surface of the sheath (Fig. 2). As has been ointed out, during the loading operation t e sheath is generally held in an upright position and the powdered insulating material is poured into the sheath through its upper end. Consequently, the reduced ortions will be located in the sheath on t e sides of the supports which will be toward the lower end of the sheath when the sheath is in its upright loading position. As shown, this lower end is at the left of the figures and thus each reduced ortion will be located at the left hand side 0 its corres onding sup ort.

Preferably an as shown iigs. 1 and 2), each reduced portion will be ositioned in the sheath a relatively short 'stance from the nearest edge of its support so as to rovide for any variation that might be ma e in the marking of the sheath or in the stretch of the resistance element. It is also preferable when locating the positions for the reduced portions 12a that the sup rts be positioned against the enlarged or attened portions of the resistance element which will be below the supports when the unit is in its upright loading position. This also makes provision for any variations that may occur in the marking of the sheath or in the stretch of the resistance element.

The resistance element with the terminals and supports is then inserted in the metallic sheath, as shown in Fig. 3, the sheath having been prepared for the'assembly by providing 8 a counterbore 22 in each of its ends and by thoroughly cleaning the interior of the sheath. The outer end of the terminal 14 is held in place centrally of the sheath by the washer 20 which is received in the left hand counter-bore 22 while the outer end of the terminal is held in place centrally of the sheath by means of the pin 21 which is received in the counterbore 22 at the right hand end of the sheath. In assembling, the resistance element will be inserted in the sheath so that the washer will be received in the left hand counterbore 22 and thereby serve as an anchor. Then the terminal 15 will be drawn or pulled out'so as to project from the right hand end of the sheath. The pin 21 will then be passed through the aperture in the flat terminal portion 15a and the terminal 15 allowed to be withdrawn into the sheath by the tension of the resistance element so that the pin will be received in the right hand counterbore 22. Thus, in

securing the terminals in place the resistance duced sections of the sheath.

element is stretched to separate its turns.

The metallic sheath is then indented or reduced to such a degree at the chalked portions that it will be impossible for the insulating supports 16 to pass through the re- It will be observed that the insulators will have been positioned upon the resistance element so that the enlarged flange-like portions 16b will be adJacent the reduced portions of the sheath. By reason of this arrangement, should an in sulati 1 support be moved so as to engage the inner walls of its reduced portion, the

. squared head will provide a free passageway through the sheath for the insulating material. Thus, it will be observed by reference to Fig. 4 that the insulating material will have a free passageway from the upper portions of the sheath to the lower portions thereof. The material may flow around the reduced cylindrical portion 16a of the supports and through the passageways provided by the squared portion 166 or it may pass through the apertures provided in the sup ports.

The sheath is now filled through the open end of the sheath, that is, the right hand end of the sheath as viewed in the figures, with the powdered heat refractory insulating material 13. To accomplish this the sheath is held in an upright position in a special vibrating machine (not shown) with the end provided with the pin 21 uppermost. As the sheath is filled with the insulating material through the open upper end, it is vibrated to shake the insulating material together and distribute it uniformly around the terminals, the resistance element and the insulating supports. As has been ointed out, thesupports have such a shape t at the insulating material isfree to pass around them as well as between the turns of the resistance element and through the centers of the supporting members. If there is any tendency for the resistance element to move lengthwise in the sheath due to the weight or pressure of the powdered insulating material either on the resistance element itself or on the supports, the supports will move into direct contact with the reduced portions of the sheath and will thus be held from further travel. If there be any tendency for the resistance element to move endwise with the powdered insulating material as it flows through the sheath, the enlarged flattened portions of the element above the supports will come in direct contact with the upper ends of the supports and thereby prevent further movement of the element. It will be understood that the supports will be sufficiently close to prevent considerablelocal movement of the element between them.

When the sheath has been filled, a washer 23 is inserted in the recess 22 at the right hand end of the sheath over the flattened terminal portion 15a and the pin 21. The ends of the sheath are now spun over so as to secure the washers 20 and 23 in place.

It has been found, heretofore, that there is a tendency for the resistance element to travel through the magnesium oxide toward the side of the sheath in the event the sheath is not perfectly straight during the swaging operation. Thus, if the operator 1n forcing the sheath through the swaging machine allowsv the outer end to sag, the sheath will curve and. as a result, itis very likely that the resistance element will cut through to the lower side of the sheath. This action, of course, reduces the clearance between the resistance element and the sheath and often-times even causes the resistance element to ground. To prevent this, I compact the insulating material about the resistance element which lies between the reduced portions 12a by compressing the sheath approximately midway between these portions of the sheath. I have found in certain instances that by compressing the sheath so as to reduce its diameter approximately and through a length of several inches between the indents, the insulating material will be compacted sufiiciently to retain the resistance element absolutely central during the swaging operation.

The entire unit is now passed through a swaging or rolling machine for the purpose of compacting the insulating material within the sheath to a hard dense mass, the swaging 0 eration reducin the diameter and increasing the length 0 the unit. It will be observed that during the swaging operation the insulating material 'will be prevented from piling up or flowing ahead of the swaging machine die both by reason of the compacted insulating portions between the reduced portions 12a and because of the insulating supports. Moreover, it will be observed that by reason of the compacted rtions of the insulating material, it will be impossible for the resistance element to travel side-wise in its sheath. Consequently, the resistance element will be firmly embedded in the compacted insulating material in a central position in the sheath.

The sheath will be reduced to a diameter equal to or less than the diameter of the reduced portions 12a. It is usually desirable to pass the unit through the swaging machine a number of times so as to reduce its diameter gradually. The unit may be annealed between the sucmssive swaging operations in order prevent the resistance elemeat from getting too hard and likewise to prevent the sheath from becoming brittle.

l have found at times that there is some tendency for the enlarged head portions 16?) or the supports to weaken the metallic sheath. In order to eliminate this trouble, I have provided each head portion with a transverse slot 160. This slot weakens the head to such an extent that the head is broken during the first swaging 0 ration. Consequently, there will be less ten ency for the head to weaken the sheath. Generally he supports 16 will he completely pulverized by the swaging operations.

After the unit has been swaged to the desired extent the washers at each end are cut of? otherwise removed and the ends of the unit ssuared off as shown in Fig. 5. The terminals are held firmly in place by the compacted insulating material in which they are embedded.

:W hat 1' claim as new and desire to secure by Letters intent of the United States is:

l. The method of making an electric heat ing unit which comprises mounting a resistance element a sheath, filling the sheath with. powdered insulating material while securing the resistance element at a plurality of points spaced at intervals along its lengt so as to prevent lengthwise movement of said element in said sheath and then compacting said insulating material aroundsaid resistance element.

2. The method of making an electric heating unit which comprises mounting a resistance element in a sheath and then loading said sheath with insulating material while securing said resistance element at a plurality oi points spaced at intervals along its length so as to prevent movement of said element in the direction of its length during the loading operation.

3. The method of making an electric heating unit which comprises mounting a coiled resistance element in a sheath, securing said resistance element against axial movement at a plurality of points spaced axially of said element and then loading said sheath with powdered insulating material introduced into said sheath at a point from which it flows substantially axially of said resistance element.

4. The method of making an electric heating unit which comprises mounting a resistance element in a sheath, then fillin said sheath with insulatin material while securin the resistance e ement at a pluralit of points spaced at intervals along its lengt so as to prevent both lengthwise and sidewise movement of said element in said sheath during the loading operation and then compacting said insulating material around said resistance element.

5. The method of making an electric heating unit which comprises mountin a coiled resistance element in a metallic sheath lengthwise of said sheath, filling said sheath through one end thereof with powdered insulating material with said coiled resistance element secured at a plurality of points spaced at intervals along its length so that said element will not be displaced longitudinally in said sheath by said insulating material as the sheath is being filled and then compacting the insulating material around the resistance element.

6. The method of making an. electric heating unit which comprises securing insulating members along the length of a sinuous resistance element, inserting the resistance element together with said members lengthwise in a metallic sheath, loading said sheath with owdered heat refractory insulating material while securing said members against endwise movement by said insulating material during the loadmg operation and then com acting said insulating material to a hard dense mass around said resistance element by reducing the sheath.

7. The method of making an electric heater which comprises securing insulating supports at intervals along the length of a helical resistance element, mounting said resistance element together with its supports lengthwise in a metallic sheath, providing said sheath with reduced sections so as to prevent movement of said supports and consequently of said resistance element in one direction, then filling said sheath with powdered insulating material so that said material flows in said one direction and then compacting said insulating material about said resistance element.

8. The method of making an electric heating unit which comprises securing insulating su ports at intervals along the length of a he ical resist-ance element, mounting said resistance element together with its supports centrally of a metallic sheath, compressing said sheath at one side of each of said an ports so as to prevent endwise motion of said supports and consequently of said element in one direction, introducing powdered insulating material through one end of said said sheath at one side of each of said supports so as to prevent endwisemotion of said supports and consequently of said element in one direction, introducing powdered insulat' ing material through one endof said sheath so that said material flows in said one direction, compacting the insulating material in those portions of said sheath between said supports and then compactin the insulating material throughout sai sheath to a hard dense mass by reducing the sheath.

10. The method of making an electric heating unit which comprises threading a plurality of insulating supports on a helical resistance element, spacing said supports along the length of said element and then flattening the coil on each side of each support so as to secure the supports, placing lengthwise of a metallic sheath, indicating said resistance element with'its supports on said sheath the positions assumed by said supportsrelative to the length of said sheath, inserting said resistance element with said supports lengthwise in said sheath with said supports in said relative positions, then reducing said sheath adjacent said positions so that said reduced portions are located on corresponding sides of said supports, introducing the powdered heat refractory insulating material into said sheath through its ends on the other sides of said supports and then compacting said insulating material by reducing said sheath.

11. The method of making I an electric heating unit which comprises threading a plurality of insulating supports on a helical V resistance element, securing terminals to the end portions of said resistance element, spacing said supports along the length of said element, flattening the turns of said element adjacent each support and on each side of said support so as to secure the support, placing said coiled resistance element with its su ports lengthwise of an elongated metalllc sheath with the element tensioned to the len hit will assume when positioned wit iin the sheath, indicating on'said sheath the positions assumed by said supports, mounting said-resistance element with its supports centrally of said sheath with one end of said sheath closed and the other end open, then reducing said sheath on the side ment of said supports and consequently of said resistance element toward said. closed end, then filling said sheath through its open end with a powdered heat refractory insulating material and then compacting said insulating material to a hard dense mass by reducing said sheath. 7 y

In witnesswhereof, I have hereunto set my hand this 26th, day of November, 1929. LORETTA M. DALY.

of each support toward the closed end of said sheath so as prevent lengthwise move- 

